According to a conventional image forming apparatus, a sheet cassette is provided which is movable between an attached position at which the cassette is attached to an attachment portion of a main housing and a detached position at which the cassette is detached (separated) from the attached position. The sheet cassette includes a sheet accommodating portion for accommodating a sheet, a lifter plate for supporting sheet(s), and a lift mechanism. The lifter plate is provided at the sheet accommodating portion and displaceable in vertical direction. The lift mechanism is configured to elevate the lifter plate.
For forming an image on the sheet in the conventional image forming apparatus, the lift mechanism is operated to lift the lifter plate to elevate a stack of sheets on the lifter plate to a sheet supplying position. Each one of the sheets lifted to the sheet supply position is separated from the sheet stack by a separation roller, and the separated sheet is conveyed toward an image forming unit by a conveyer roller. To this effect, the lift mechanism is driven by meshing engagement with a drive gear to which a drive force from a drive source is transmitted, so that the lifter plate can be elevated.
Here, the drive gear and the conveyer roller are exposed to a space in the sheet attachment portion. Therefore, if the drive gear and the conveyer roller are driven while the sheet cassette is at the detached position after the sheet cassette is detached from the attachment portion, foreign objects entered into the attachment portion from an outside may be trapped onto the drive gear and the conveyer roller, impeding normal image forming operation.
In order to avoid this drawback, Japanese Patent Application publication No. 2011-195319 discloses an image forming apparatus provided with a clutch provided in a gear train that transmits drive force from the drive source to the conveyer roller. The clutch is adapted for shutting off the drive force transmission to the conveyer roller in order to stop driving of the conveyer roller when the sheet cassette is at the detached position.